1. Field of the Invention
This invention relates generally to rotating electric machinery and particularly to gas turbine engines.
2. Description of the Prior Art
A gas turbine engine constructed in accordance with prior art technology transfers power extracted by the turbine to a compressor by means of a mechanical shaft. In steady state operation this requires that the physical speed of the compressor and turbine be identical unless gearing is used. If several spools are used, the power and speed must be matched for each shaft, which imposes strict design constraints and limits the engine's operability.
Several problems, such as low component efficiency, rotating stall and surge, etc., can arise in a gas turbine during operation at off-design conditions and transient conditions. These problems are known to occur frequently while a gas turbine is starting up. Start-up problems are mainly due to the stages not being properly matched, which creates large negative and/or positive airfoil incidences at various blade rows through the engine. The rotational speed and power matching constraints of the turbine(s) and compressor(s) are two of the factors that create this problem.
Multistage turbomachines, such as compressors or turbines, are usually made of alternating rotating (rotor) and stationary (stator) rows of airfoils or similar turning elements. The rotor blade elements add (compressor) or extract (turbine) energy from the working gas while the static rows simply redirect the flow for the next rotating row. The presence of the stator rows, which do not contribute to the energy exchange, reduces the efficiency and increases the size, particularly the length, of the component. Eliminating these rows can increase turbomachine stage efficiencies from approximately 90% to 95% and significantly reduce the component length.
In many applications of gas turbines, such as electric power production or propulsion (non-exhaust gas driven), a separate generator is used to convert the shaft power to electric power, which requires a linkage between the shaft and the generator. This can significantly increase the overall length of the gas turbine/generator assembly.
Ordinarily a separate starter mechanism is required for a gas turbine. In order to start a gas turbine the starter mechanism is necessary to supply power to the shaft. The starter mechanism is then disengaged after self-sustaining operation is obtained.
In a typical gas turbine, the compressor and turbine are co-linear due to the shafting. For some applications (such as ship-board installation) it could be advantageous to relax this constraint.
Several techniques are used to improve starting and off design performance of gas turbines including using multiple shafts (typically two or three), interstage compressor bleeds, and variable stator geometry. Variable rotor geometry has also been proposed.
Counter-rotating spools are sometimes used to eliminate the intermediate stator. This increases the efficiency, power to weight ratio and reduces cooling flows in turbines. However, many shafts (with great mechanical complexity) are necessary to eliminate all the stators in a high-pressure ratio machine.